The fourth content area from my whirlwind overview of DAMOP is “traditional” AMO physics. This was the hardest to talk about in my talk, because I know it the least well, but ironically, that makes it really easy to write up here, because I don’t have much to say about it.
Where the other areas were largely about using atomic, molecular, and optical physics to do stuff (simulating condensed matter systems, generating coherent x-rays, demonstrating cool quantum effects), this sub-sub-field is concerned with directly investigating the properties of atoms and molecules, usually by bouncing other things (other atoms, electrons, photons) off them.
The most traditional of traditional areas is straight-up spectroscopy, where you look at the colors of light absorbed and emitted by the systems of interest. A lot of people think this sort of physics ended about 50 years ago, since you can look up most spectral lines in books of tables or handy reference sites on the Internet), but in fact, there are still people working on refining the values of the wavelengths absorbed and emitted by various atoms and molecules. A number of “known” values turn out to be off by a surprising amount, so it’s important to check these as techniques improve.
The other areas tend to involve substantially similar apparatus, which I will illustrate via a pretty picture taken from this paper in Science (paywalled, alas):
Generally you have a beam of atoms or molecules that are all headed in more or less the same direction, and at some point you combine that beam with something else– a laser beam in the figure above, or a beam of some other kind of atoms or molecules, or an electron beam. By looking at what comes out of the region where the beams interact, you learn something about the target system in your original beam.
What kind of things do you learn? It depends on what you’re after. A lot of these experiments deal with photoionization or photodissociation, where you either blast electrons off an atom or blow a molecule into pieces with a laser. This tells you fairly directly about the energy level structure of the target atoms or molecules.
Other experiments are basically doing chemistry, like the one from which I lifted that figure. They combine a couple of different types of atoms and molecules, and look at how they re-arrange themselves. The measured quantities here are collision cross-sections and reaction rates.
Why is this interesting? The data obtained from these experiments are crucially important to studies of atmospheric physics, leading to my favorite title from the DAMOP prgoram book, Why isn’t the atmosphere completely ionized? It’s actually a good question, as the atmosphere is constantly bombarded by cosmic rays, which tend to ionize things. Studies of neutralization reactions in the laboratory tell us something about what’s going on in the upper atmosphere.
The other big area of interest is astrophysics, which is why the paper I linked above got into Science— they measured some reaction rates for formation of hydrogen molecules, which have implications for astrophysics. By learning about how hydrogen molecules react, we learn about processes involved in the formation of early stars and galaxies (link to a commentary on the original article, which is probably also paywalled, alas).
So, what did I highlight in my talk? Well, I copped out a bit, and went with the session on antihydrogen (which was trapped last year, and has now been trapped for 1000 seconds or longer). This might seem more like a Dan Brown novel than AMO physics, but the fundamental processes involved are a very traditional AMO sort of thing: positron capture by an antiproton isn’t significantly different than electron capture by a proton.
Names to Conjure With: My knowledge of this part of the field is so sketchy I won’t insult the people involved by screwing up a list of prominent researchers to follow. I know a handful of people in this area, but not enough to claim any kind of authority.